The present invention is directed to a circuit for phase section control or phase control of an AC-DC motor for a processing or utility machine operated in defined load conditions. If the AC-DC motor of a processing machine, such as a circular saw, drilling machine, a hammerdrill or the like, is operated directly from the power supply without electronic control, then the motor rpm in the no-load condition is about 20 to 40% higher than under load. With each load change in both directions, therefore, from the idling state into the operating state under load and vice versa, rpm changes of the above-mentioned order of magnitude occur. In certain machines, such as in the case of electropneumatic drill, hammers, the large rpm change from idling to loading is particularly critical, since very high pressure peaks occur at high velocities of the power piston with the consequence of disproportionately high wear and other disadvantages when using the device. If, on the other hand, the motor is equipped with an rpm control, then the voltage of the motor as a function of a standard deviation is so adjusted by an output adjustment link or final control element that the rpm deviation becomes a minimum. Hereby, however, the rpm must be continuously measured and the entire circuitry becomes comparatively large.
A further possibility consists in operating the motor through a power adjustment link or final control element so that the motor is supplied on the input side by means of an adjustment or control value originating from the measurement of the motor current, thus, not from a regulator. With increasing load, the exact amount of voltage is supplied additionally to the power adjustment link through the current increase connected with the increase in load, so that the rpm remains constant. The interrelationships between load, current rpm and voltage are, however, non-linear to such an extent across the ranges that have to be controlled in actual practice, that either one has to expect very large errors in rpm or the circuit begins to oscillate in certain rpm ranges.
A further possibility consists in equipping the motor with a final control element adjustable by the user. Because of erroneous operation which is easily possible, no effective rpm control in the case of load changes is possible with this.
The invention, therefore, has as an object, the improvement of the control of an AC-DC motor supply over an output final control element in such a way that largely the same operational rpm is maintained under load conditions occurring in actual practice.
The inventive solution results in a circuit for phase control of an AC-DC motor for a machine operated under defined load conditions, as set forth in the appended claims.